US6426836B2 - Method and apparatus for reducing the formation of spots in laser projection - Google Patents

Abstract

A projection surface includes scattering microparticles that are in constant motion in relation to each other and to a source of laser radiation in a laser projection system. The microparticles reduce the formation of spots that normally occur when viewing the contents of images which are uniformly illuminated by means of laser projection and which are reflected off a projection surface. The microparticles are reflective in a suspension fluid and move due to Brownian molecular motion. A mirror is placed on the back wall of the screen so that the viewer sees both the backscattered light from the incident beam as well as the forward scattered light from the reflected beam. A cellular structure is located in the suspension fluid to laterally confine the scattered light increasing the sharpness of the laser light seen by the viewer.

Description

This application is a continuation of Ser. No. 08/933,185, filed Sep. 18, 1997, now abandoned, which is a continuation of Ser. No. 08/873,209, filed Jun. 11, 1997 now U.S. Pat. No. 6,092,900.

BACKGROUND OF THE INVENTION

This invention relates to a system for improving the image brightness and image sharpness in laser television projection systems.

Laser projection uses a raster procedure to produce images. In the raster procedure, three primary colors red, green, and blue are emitted from one laser or from two or more different lasers. The three colors are modulated separately, brought together again, and scanned over a surface. The image is then formed by points, as in the standard television picture tube. The color of an image point is defined by the relative power of its primary color components. Normally the beam is deflected horizontally by a polygonal mirror and vertically by a galvanometric scanner.

The laser beam causes image irregularities (spots) or so-called speckles in the laser image projection due to the spatial coherence of the laser light. The effect can be observed by shining a diverged laser beam onto a wall. The eye of a viewer sees a disk with a stochastic light-dark distribution, instead of a uniformly illuminated field.

The spots are perceived when light is reflected off a projection surface and to the eye. The projection surface has areas which are uneven compared with the size of the light source wavelength. The light waves reflected off these uneven surface areas of the projection surface reach the eye with different phase positions creating interference that is perceived in the eye. From a given standpoint of the observer, this interference produces a spatial two-dimensional light-dark pattern.

A laser projection system is described in European Patent No. EP0589179A1 where laser radiation consisting of three primary colors is passed through a diffusion element in a common optical path. The diffusion element rotates in the range from 30 to 40 rpm to prevent the formation of spots due to interference phenomena and creates a clearer, truer-color image.

A projection light image display system with reduced spot formation is also described in European Patent No. EP0385706. A coherent light source is used to produce a light beam. A light modulator directs the light from the source onto the display screen reproducing an image. The screen is coupled with a transducer that creates acoustic surface waves in the screen where the image is reproduced. The acoustic waves possess an amplitude which is greater than the wavelength of the light beam. The surface waves are supposed to prevent the formation of spots when viewing uniform image contents taking advantage of the fact that the eye is not quick enough to perceive the moving interference pattern created by the surface waves.

A system is also described in U.S. Pat. No. 5,313,479, in which a rotating diffusion element is arranged in the optical path of the laser. The diffusion element moves the interference pattern so quickly that it cannot be perceived by the human eye.

The systems described above require mechanical transducers and, therefore, are relatively expensive. The diffusion elements mentioned above also induce lateral diffusion of photons in the screen which reduce image sharpness and brightness. If the reflectivity of a laser projection screen is low, more powerful lasers are needed which are more costly, require more power and cooling and have a shorter operating life.

Thus, a need exists for reducing interference in light image display systems while at the same time maintaining high image sharpness and brightness.

SUMMARY OF THE INVENTION

A projection surface reduces the formation of spots during laser projection while increasing the sharpness and brightness of the laser projection after being scattered by the screen. The projection surface includes scattering microparticles that are in constant motion relative to each other and a source of laser radiation of the laser projection. The microparticles reduce the formation of spots that normally occurs when viewing the contents of images which are uniformly illuminated by means of laser projection and which are reflected off a projection surface.

The projection surface is made in such a way that although the image irregularities (spots) or speckles still occur, they are variable in time so that the two-dimensional light-dark pattern, when seen over the reaction time of the eye, is averaged out. Thus, the viewer perceives a uniform brightness.

In one embodiment of the invention, the microparticles are formed in a suspension fluid. Due to Brownian molecular motion in the fluid, the microparticles continue to move. The fluid may be warmed to further increase motion of the microparticles. In another embodiment of the invention, a piezoelectric force is used to further increase motion by causing turbulence in the microparticles. It is also advantageous to constantly mix the particles to prevent settling. Both embodiments are relatively simple and are resistant to interference.

In place of a solid projection wall, the projection surface has mobile scattering centers. These scattering centers are implemented by suspending the microparticles in the fluid. The motion of the microparticles creates a large number of different light/dark patterns during illumination by the laser spot. The eye perceives the patterns as a uniform image for uniform image contents.

In another embodiment of the invention, the back wall of the screen is made reflective. The incident laser beam passes through a front window and through the suspension fluid that backscatters the light to remove stationary speckle. The light is also reflected back after hitting the back mirror. The viewer sees both the backscattered light from the incident beam as well as the forward scattered light from the reflected beam. The reflected light gives rise to further scatter and thus adds to image brightness.

In another embodiment of the invention, a cellular structure is located in the suspension fluid. The cellular structure laterally confines light increasing the sharpness of the laser light seen by the viewer. The cells can be different shapes such as, honeycomb shaped or square shaped. Different absorbing, scattering or reflective surfaces can be used on the sides of the cellular structure in contact with the suspension medium according to the desired brightness and sharpness of the laser projection.

The foregoing and other objects, features and advantages of the invention will become more readily apparent from the following detailed description of a preferred embodiment of the invention which proceeds with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1ashows a schematic top view of a laser projection arrangement with a fluid projection surface according to the invention.

FIG. 1bshows a schematic longitudinal sectional view of the laser projection arrangement shown in FIG. 1a.

FIG. 2 is a schematic longitudinal sectional view of a mirror added to the laser projection arrangement shown in FIG. 1a.

FIG. 3 is a schematic longitudinal sectional view of a cellular structure added to the laser projection arrangement shown in FIG.2.

FIG. 4 is a schematic front view of the cellular structure shown in FIG.3.

DETAILED DESCRIPTION

Referring to FIGS. 1aand1b, the laser projection system according to the invention includes aprojection device1 that has three coherent sources ofradiation beam3 for the primary colors red, blue, and green in the form of lasers, as is described in U.S. Pat. No. 5,313,479 to Florence. Thelaser radiation beam3 exits theprojection device1 through anexit opening2 and strikes aprojection surface5. Thelaser radiation beam3 is deflected in the azimuth range of angle α by means of a polygonal mirror guided around a vertical axis (not shown), and in height in the range of angle β by galvanometric scanners (not shown).

The azimuth angle α is typically within the range of 30-90 degrees and the height angle β is typically in the range of 25-50 degrees. Means of varying the azimuth angle and height angle using polygonal mirrors and galvanometric scanners, respectively, are known to those skilled in the art and are, therefore, not described in detail.

Theprojection surface5 is provided within ahousing7. A transmittingboundary surface6 resides on a side facing the projection device radiation source. The transmittingboundary surface6 andhousing7 are shown schematically and contain asuspension fluid8 withlight scattering particles9. Thescattering particles9 scatter thelaser beam3 entering through theboundary surface6. The motion of theparticles9 prevents the formation of an interference pattern. The path length changes due to the motion ofparticles9 are greater than the wavelength of the reflected light. Thus, the eye of a viewer does not perceive changes in brightness of thescattered laser beam3.

The largest average diameter of theparticles9 is on the order of 100 μm or less down to the order of nanometers. Due to Brownian motion, thesuspension fluid8 constantly moves at room temperature sufficiently to prevent interference patterns. An alternative embodiment, heatssuspension fluid8 withadditional heating resistors11 to increase the Brownian motion and to further reduce spot formation in the eye of the viewer when viewing theprojection surface5.

The motion of theparticles9 in thesuspension fluid8 is also increased by usingpiezo oscillators12 acting on one or several membrane that each comprise scattering particles in a suspension material. Thepiezo oscillators12 force even mixing of the scattering particles in the suspension fluid. The deflection path of theparticles9 caused by the effect of the piezoelectric force is also above the wavelength of the reflectedlaser light3.

FIGS. 1aand1bshow theprojection surface5 in a top sectional view and a longitudinal sectional view, respectively. The longitudinal sectional view in FIG. 1bshows a projection surface suitable for motion picture/television or video purposes. The projection housing of thelaser projection device1 should ideally be located not too far above the height of the viewer so the scattered light is headed toward the eyes of the viewer. The radiation fromlaser light3 passes through the transmittingboundary surface6 and is reflected by theprojection surface5 made up oflight scattering particles9. The layer ofsuspension fluid8 can be different thicknesses. However, a thickness between 0.1 centimeters and 2.0 centimeters provides sufficient scattering and also reduced weight.

One embodiment of the invention uses milk as theprojection surface5. The milk contains coagulated protein andfat particles9 suspended inwater8. The milk is illuminated by thelaser radiation3. A spot observed on the surface of the milk produces more uniform brightness compared with the same laser spot when shone onto solid cardboard.

The medium causing the required back-scatter could be a suspension of glass spheres of very small particles such as aluminum oxide particles or silicon oxide particles suspended in, e.g., water or oil. Thesuspension material8 can alternatively be a gas or any alternative material that can evenly suspend scattering particles. Mechanical and electronic devices, other than thepiezoelectric transducer12, can be used to keep thescattering particles9 evenly mixed in thesuspension material8.

The scatter length over which all but a fraction of theincoming laser beam3 is scattered should not be large compared to the depth of the cell in order to have good power efficiency. This is controlled by particle concentration in thesuspension material8 or by choosing the cell depth accordingly. The function of theliquid8 is to move the particles sufficiently by diffusion over the retinal integration time. Also, it is advantageous to make the scatter length comparable to or smaller than the desired lateral resolution so that photon diffusion, as opposed to particle diffusion, does not cause image blur. The angular distribution of scattered light is optimized by controlling the shape and size (distribution) of thescattering particles9. The refractive index/indices of the different media making up the emulsion/suspension material can also be varied to optimize the angular distribution of the scattered light.

Another embodiment of the invention is shown in FIG. 2. Ascreen housing7 includes a front glass window16 and a mirrored surface14 located on a back substrate15. Theincident laser beam3 passes through the front window16 and into thesuspension material8. Thesuspension material8 backscatters the laser beam light reducing stationary speckle. Thelaser beam3 is also reflected by the mirror14, giving rise to further scatter that increases the brightness of the laser light image. Thus, when illuminated and viewed from a front side, the reflective mirror14 on the back side of thehousing7 allows a person viewing theprojection surface5 to see both the backscattered light19 from theincident laser beam3 as well as forward scattered light18 from thelaser beam3 reflected off the mirror14.

The amount of reflectivity of the back mirror14 can be varied according to the desired brightness of the laser light image visually observed on thefront boundary surface6. For example, a less reflective material will reduce brightness and a more reflective material will increase brightness.

Another embodiment of the invention is shown in FIG. 3. Acellular structure20 is located in thesuspension material8 between the front glass16 and the rear mirror14. Themulti-cell structure20 is bonded between the glass16 and the mirror14 with an adhesive. Alternatively, themulti-cell structure20 is located in ahousing7 similar to that shown in FIGS. 1A and 1B that does not include a back mirror14.

FIG. 4 is an enlarged schematic diagram of a squarecellular structure20. Theindividual cells24 are formed by thecellular structure20 and each retain some of thescattering particles9. Thecellular structure20 confines lateral scattering of the incident laser light19 toindividual cells24 improving sharpness of the viewed laser light image. The diameter of eachindividual cell24 is preferably the same or smaller than the diameter of the individual pixels displayed on theprojection surface5. This prevents thelaser light3 from different pixels from bleeding into each other without having to align thecellular structure20 with the display locations of the individual pixels.

Thecellular structure20 has either absorbing, scattering orreflective sides22 that contact with thesuspension material8. For example, thesides22 are black for light absorption or an aluminum or silver color for higher reflectivity. Thecellular structure20 can be made out of a variety of materials such as plastic, metal, synthetic resin or a fibrous sheet. Thecellular structure20 can be different shapes, such as the square shape shown in FIG. 4, a honeycomb shape, a circular shape or any shape preferable in a particular application.

Cellular structures similar tocellular structure20 are used in the drawing toy “Magna Doodle” produced by Tyco Industries, Mt. Laurel, N.J. 08054, and is described is U.S. Pat. No. 4,143,472.

Having described and illustrated the principles of the invention in a preferred embodiment thereof, it should be apparent that the invention can be modified in arrangement and detail without departing from such principles. I claim all modifications and variation coming within the spirit and scope of the following claims.

Claims (20)

What is claimed is:

1. A projection surface for reducing interference patterns perceived in a laser light image transmitted from a laser radiation source in a laser projection system, comprising:

a suspension material that includes multiple light scattering microparticles constantly in motion relative to each other and to the laser radiation source of the laser projection system, the suspension material while scattering the laser light image varying the interference patterns in time thereby preventing perception of the interference patterns; and

a mirror located adjacent to the suspension material reflecting back the laser light passing through the suspension material thereby increasing laser light image brightness.

2. A projection surface according toclaim 1 including a cellular structure located in the suspension material.

3. A projection surface according toclaim 2 wherein the cellular structure comprises a plurality of honeycomb cells or square cells.

4. A projection surface according toclaim 2 wherein the cells in the cellular structure each have a width less than a diameter for individual pixels of the laser light image transmitted onto the projection surface such that the laser light from different pixels is prevented from bleeding into each other without having to align the cellular structure.

5. A projection surface according toclaim 2 wherein the cellular structure is made of a metal, plastic, synthetic resin or fibrous sheet.

6. A projection surface according toclaim 2 wherein the cellular structure includes sides that primarily reflect light.

7. A projection surface according toclaim 2 wherein the cellular structure includes sides comprising mirrors.

8. A projection surface according toclaim 2 wherein the cellular structure includes sides that primarily absorb light.

9. A projection surface according toclaim 2 wherein the suspension material comprises a coagulated protein.

10. A projection surface according toclaim 1 wherein the suspension material comprises a fluid and a constant motion of the microparticles is produced by Brownian motion.

11. A projection system, comprising:

a projection device transmitting images with a light source;

a projection screen including scattering microparticles suspended in a suspension material and moving in relation to the light source, the microparticles deflecting light from the transmitted images thereby reducing perceived phase interference; and

a cellular structure located in the suspension material for confining lateral deflection of the light thereby increasing the sharpness of the transmitted images on the projection screen.

12. A projection system according toclaim 11 wherein the projection screen includes a mirror located in back of the suspension material that reflects back the light passing through the suspension material.

13. A projection system according toclaim 11 wherein the suspension material comprises a coagulated protein.

14. A projection system according toclaim 11 wherein width of the cells in the cellular structure is smaller than a diameter of individual pixels of the images transmitted onto the projection screen such that the laser light from different pixels is prevented from bleeding into each other without having to align the cellular structure.

15. A method for projecting images with a laser light source onto a projection screen, comprising:

suspending light scattering microparticles in a projection screen suspension material;

moving the microparticles in relation to each other and the laser light source;

deflecting the laser light off of the scattering microparticles in the projection screen for reducing perceived phase interference in the laser light;

reflecting back the deflected laser light from a back side of the projection screen to increase the brightness of the deflected laser light; and

laterally confining the deflected laser light confining the microparticles in multiple cells in the projection screen.

16. A method according toclaim 15 including increasing the movement of the microparticles by heating the projection screen.

17. A method according toclaim 15 including increasing the movement of the microparticles by applying a piezoelectric force or other mixing mechanism to the projection screen.

18. A method for projecting images with a laser light source onto a projection screen, comprising:

suspending light scattering microparticles in a projection screen suspension material;

moving the microparticles in relation to each other and the laser light source;

deflecting the laser light off of the scattering microparticles in the projection screen for reducing perceived phase interference in the laser light; and

reflecting back the deflected laser light from a back side of the projection screen to increase the brightness of the deflected laser light or laterally confining the deflected laser light confining the microparticles in multiple cells in the projection screen.

19. A method according toclaim 18 including increasing the movement of the microparticles by heating the projection screen.

20. A method according toclaim 18 including increasing the movement of the microparticles by applying a piezoelectric force or other mixing mechanism to the projection screen.

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